/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int left, int right) {
                ListNode *dummy=new ListNode(0);
                dummy->next=head;

                ListNode *pre =dummy;
                for(int i =0 ;i<left-1;i++)pre =pre->next;
                ListNode *cur = pre->next;
                for(int i = 0;i<right-left;i++){
                    ListNode *next=cur->next;
                    cur->next=next->next;
                    next->next=pre->next;
                    pre->next=next;
                }
                return dummy->next;
    }
};